1. Field of the Invention
The present invention relates to a valve nozzle of an injection molding machine for injection-molding a molding compound into a mold. More particularly, it relates to a valve nozzle of an injection molding machine in a hot runner system.
2. Description of the Prior Arts
An injection molding machine for injection-molding a molding compound made of a synthetic resin such as a thermoplastic resin or a thermosettable resin, has a nozzle at the end. A hot runner system has been known as a method of injection-molding without using a sprue or a runner of a mold. A valve nozzle has been known as a nozzle equipped at the end of the hot runner system. The valve nozzle has been used for preventing a leakage of an injection molding compound from a gate during the time of non-injection of the molding compound and for preventing a reverse flow of an injection molding compound from a cavity when the pressure for the injection molding is reduced.
The typical valve nozzle is a valve nozzle equipped with a needle valve. The systems for actuating the needle valve of the valve nozzle include a system for actuating it by an external power mechanism and a system for actuating it by the pressure for the injection molding.
The former system has disadvantages of a complicated mechanism causing many faults. Thus, the latter system is considered to be superior to the former system.
The latter system has been proposed in Japanese Unexamined Patent Publication 158654/1975 wherein a needle valve is opened and closed by a piston sliding in a cylinder as a nozzle body.
FIG. 1 is a schematic sectional view of the nozzle having the needle valve.
The nozzle comprises a cylinder (1) as a nozzle body; a piston (2) for sliding in the cylinder (1) and a needle valve (3) connecting to the piston (2). In the piston (2), the sectional area at the rear end (4) of the piston in the down-flow of the injection molding compound is smaller than a sectional area of the front end (5). Thus, in the inner diameter of the cylinder (2), the inner diameter at the rear end (6) is smaller than the inner diameter at the front end (7). The piston (2) has a passage (8) for flowing the injection molding compound in the piston and is pushed to the flow direction of the injection molding compound by a spring (9). The gate (10) of the needle valve (10) at the end of the nozzle is opened and closed by shifting the piston (2).
In the valve nozzle having the above-described structure, the injection molding compound is fed from the right side into the passage in the piston to fill it in the nozzle and to cause a pressure difference to the piston by the difference of the sectional area at the rear end from that of the front end of the piston and the piston is shifted to the right side when the inner pressure difference is increased over the pushing force to the piston whereby the gate of the needle valve is opened to actuate the valve nozzle. On the contrary, when the pressure for the injection molding is reduced under the condition of the opening of the gate, the inner pressure difference to the piston is decreased less than the pushing force of the spring, and accordingly, the piston is shifted to the left direction to close the gate of the needle valve.
The valve nozzle is advantageous in view of the automatic opening and closing depending upon the change of the pressure for the injection molding. However, it is not satisfactory. As one problem, the shift of the piston is not so fast as expected. When the injection molding compound has low melt viscosity such as a melted polystyrene, the operation is substantially satisfactory. However, when it has high melt viscosity such as polycarbonates and fiber reinforced resins reinforced with a filler or a glass fiber, the operation is not satisfactorily fast, because the pressure loss in the passage in the piston or on the inner wall of the cylinder is too high and the pressure at the front end of the piston is too low.
The satisfactory pressure difference over the pushing force of the spring is not resulted and the piston is not shifted unless the difference of the sectional areas of the piston is effectively utilized to proportion the pressure at the front end of the piston to the pressure at the rear end of the piston. Even though the piston is shifted to open the gate and the injection molding compound is flowed, it is difficult to maintain the pressure difference for preventing the shifting of the piston against the flow of the molding compound.
As the second problem, when the reverse flow pressure of the injection molding compound injected into the cavity is increased, there is high possibility to cause the reverse flow of the injection molding compound into the nozzle by opening the gate.
In the injection compressing molding for a compress-molding of the molding compound injected into the cavity, the reverse flow pressure is remarkably high and the needle valve is pushed by the molding compound to open the gate whereby the reverse flow is easily caused. When the pushing force of the spring is increased to prevent the reverse flow, the difficulty of the operation of the valve is caused because of the former problem.
When the sectional area of the gate is decreased to prevent the reverse flow, the following third problem may be caused. Thus, the problem is not dissolved by it.
As the third problem, the sectional area of the gate at the end of the nozzle can not be increased. In order to complete the injection for a short time, it is necessary to inject a large amount of the injection molding compound for a short time.
In the valve nozzle having the needle valve, the sectional area of the gate can not be increased. Even though the sectional area of the gate can be increased by certain manner, a gate mar remained on the product or a diameter of the spool are large and accordingly the cutting or the grinding these parts are not easy.